1. Field of the Invention
The present invention relates to a light source device, an optical scanning device that includes the light source device, and an image forming apparatus that includes the optical scanning device.
2. Description of the Related Art
Optical scanning devices relating to laser printers or the like are widely known. Generally, in such an optical scanning device, a light beams emitted from a light source are deflected by a deflector. Through an optical scanning and imaging system such as an fθ lens, the light beams are directed to and focused on a surface to be scanned (hereinafter, “scanned surface”). Accordingly, a light spot is formed on the surface to scan the surface in the main-scanning direction. The surface is, for example, a photoconductive surface of a photoreceptor of an electronic imaging-forming apparatus.
An example of full-color image forming apparatuses includes four photoreceptors arranged along the direction in which a recording sheet is transferred, a plurality of light source devices that corresponds to the photoreceptors, and a plurality of optical scanning and imaging systems that corresponds to the photoreceptors. Light beams emitted from the light source devices are deflected by a deflector and then simultaneously scan scanned surfaces of the photoreceptors through the optical scanning and imaging systems. Accordingly, an electrostatic latent image is formed on each of the scanned surfaces. The electrostatic latent images are visualized by an image developer using developers of different colors such as yellow, magenta, cyan, and black. Thereafter, the resultant images are sequentially transferred onto a single recording sheet so that the images overlap, and then are stabilized. In this manner, a color image is obtained.
The imaging forming apparatus that includes at least two combinations of optical scanning devices and photoreceptors to obtain a two-color image, multi-color image or full-color image is known as tandem-type image forming apparatuses. The tandem-type image forming apparatus may include a single deflector that corresponds to all of a plurality of photoreceptors.
With respect to tandem-type image forming apparatuses each including a single deflector, for example, Japanese Patent No. 3295281 discloses an optical scanning device that includes a deflector and a plurality of optical scanning units that is arranged along the sub-scanning direction. The light beams substantially parallel and separated in the sub-scanning direction are incident on the deflector, and then, each of the light beams passes through a corresponding one of the optical scanning units to scan a corresponding one of scanned surfaces. Japanese Patent Application Laid-open Nos. 2001-4948, 2001-10107, and 2001-33720 disclose a technology in which a plurality of light beams directed toward different scanned surfaces is incident on one side of the deflector. After the light beams pass through lenses L1 and L2, each of the light beams passes through corresponding one of lenses L3.
When a single deflator that deflects a plurality of light beams is used instead of a plurality of deflectors, an image forming apparatus can be miniaturized.
The number of deflectors of an optical scanning device can be reduced in a full-color image forming apparatus that includes four scanned surfaces (photoreceptors) of different colors such as cyan, magenta, yellow, and black. However, the size of the deflector such as a polygon mirror increases in the sub-scanning direction because light beams directed to the photoreceptors are substantially parallel and separated in the sub-scanning direction when being incident on the deflector. In general, a polygon mirror costs high compared to other optical elements that constitute an optical scanning device. For such reasons, cost reduction and miniaturization of the optical scanning device are difficult.
For cost reduction, Japanese Patent Application Laid-open No. 2003-5114 discloses an optical scanning device of a color image forming apparatus having an oblique-incidence optical system that includes a single deflector. The deflector has a deflecting-reflecting surface on which each of a plurality of light beams is incident at an angle with respect to the sub-scanning direction.
In the oblique-incidence optical system, light beams are deflected and reflected on the deflecting-reflecting surface. Each of the light beams is directed to a corresponding one of the scanned surfaces through a corresponding one of deflecting mirrors. The angles, at which the light beams are incident on the deflecting-reflecting surface, allow the light beams to be separated by the deflecting mirrors.
The oblique-incidence optical system assures separation of adjacent light beams directed to the different scanned surfaces without an increase of the size of the deflector. In other words, an increase in the number of polygon mirrors and the thickness of the polygon mirror.
However, in the oblique-incidence optical system, large scanning line curvature may occur. The amount of the scanning line curvature varies depending on the angle of each light beam to the sub-scanning direction, at which the light beam is incident on the deflecting-reflecting surface. The curvature causes a difference in color of an image obtained by overlapping electrostatic latent images formed by the light beams and visualizing them using different color toners. The light beams are obliquely incident on a scanning lens, which increases wavefront aberration and deteriorates optical performance at a peripheral image height. Thus, a beam-spot diameter increases, which lowers image quality.
As a method of correcting the large scanning line curvature caused in the oblique-incidence optical system, for example, Japanese Patent Application Laid-open No. H11-14932 discloses an optical scanning and imaging system in which a lens-surface tilts in a sub-scanning direction is changed in a main-scanning direction to correct scanning line curvature. Japanese Patent Application Laid-Open No. H11-38348 discloses an optical scanning and imaging system in which a reflection-surface tilts in a sub-scanning direction is changed in a main-scanning direction to correct scanning line curvature.
Japanese Patent Application Laid-Open No. 2004-70109 discloses a technology in which a light beam obliquely incident on a deflector passes outside the axis of a scanning lens, and scanning lines are aligned with a surface by which the amount of asphericity of the non-generatrix of the scanning lens changes along the main-scanning direction. With the conventional technology, scanning line curvature can be corrected with a single scanning lens. However, Japanese Patent Application Laid-Open No. 2004-70109 does not refer to an increase in beam-spot diameter resulting from increase of wavefront aberration.
The skew rays of the oblique-incidence optical system tend to increase wavefront aberration, and thus, increase a beam-spot diameter near the ends of a scanning line. To perform scanning with beam spots in high density, the beam spots need to be prevented from being larger. While the conventional technologies are capable of correcting the scanning line curvature, the wavefront aberration is not sufficiently corrected.
As an optical scanning device for correcting both scanning line curvature and wavefront aberration, Japanese Patent No. 3450653 discloses an optical scanning device that includes an optical scanning and imaging system that includes a plurality of asymmetric-surface lenses. The shape of a generatrix that connects the vertices of non-generatrices on the lens surfaces of the rotating asymmetric lenses is curved in the sub-scanning direction.
However, because each of the lenses corresponds to each of light beams, an increased number of lenses are necessary when the optical scanning and imaging system is used for a tandem-type optical scanning device.
When a plurality of light beams directed to different scanned surfaces is incident on the same lens, scanning line curvature and wavefront aberration of one of the light beams can be prevented by curving the generatrix. However, it is difficult to reduce scanning line curvature and wavefront aberration caused by the other light beams.
Because of curvature of a lens in the sub-scanning direction, light beams incident on the lens may shift or tilt in the sub-scanning direction due to an error that occurs when the optical elements are processed or assembled. In such a case, the scanning line curvature is caused from refraction of the lens. Accordingly, prevention of color difference that is originally thought to be possible cannot be achieved.
It is also difficult to prevent wavefront aberration and obtain a preferred beam-spot diameter stably because the angles of the beams on the curved surface vary and thus the amount of skew of the light beams changes. Accordingly, the image quality deteriorates.
In addition, when the angles of incidence are changed as described, the light beams may not precisely pass through or be deflected by other optical elements such as scanning lenses or deflecting mirrors. Consequently, the beam-spot diameter is changed, and further, the light beams are prevented from reaching the scanned surfaces.
The above-described inconveniences are solved by accurately processing and assembling the optical elements. However, the accurate processing and assembly increase the cost of the optical elements and assembly time.
When the angles of incidence of the light beams on the deflecting-reflecting surface are increased in the oblique-incidence optical system, various types of aberrations are increased, which deteriorates optical performance. Specifically, the beam-spot diameter changes and the scanning line curvature increases. For this reason, it is preferable that the angles of incidence be smaller.
However, smaller angles of incidence make it difficult to cause each of light beams to be directed to a corresponding one of scanned surfaces. The light beams are separated with the minimum intervals at a separation point such that the light beams are directed to and incident on different deflecting-reflecting surfaces at smaller angles. However, when the angles of incidence are changed, some of the light beams may not precisely pass through or be deflected by the optical elements, and the beam-spot diameter increases. Even when the light beams precisely pass through or are deflected by the optical elements, the beam-spot diameter changes, and preferred beam spots cannot be obtained.
Japanese Patent Application Laid-open No. 2004-271906 discloses a light source device of,an oblique-incidence optical system. However, the light source device cannot prevent the change in angles of incidence due to the processing error or assembly error. Coupling lens can be adjusted in the sub-scanning direction after all optical elements are assembled. However, in this case, attachment or adjustment of the light source device is complicated, and the time required for adjustment increases.